Zywave aberrometer

There are two common ways of measuring wavefront aberrations. The first method is to measure the actual aberration of the wavefront itself from an ideal reference wavefront. This method is typically performed using interferometric techniques when measuring optical surfaces or lens elements, or in high-energy laser technologies.

The second method involves reconstruction of the wavefront by measuring the localized slope at many positions across the wavefront. The most common technique used for ophthalmic applications to reconstruct the wavefront is borrowed from the world of astronomy and is called the Shack-Hartmann technique after its inventors. This method uses an array of microlenses to capture information about the local slope of the wavefront exiting the eye at dozens of locations across the pupil. An extremely small-diameter infrared laser beam is focused onto the retina. The small size of the beam ensures that the aberrations of the eye do not affect the light coming into the eye. The small spot on the retina acts as a point source emitting light rays out of the eye. These rays are affected by each of the optical elements in the ocular system and contain the entire aberrations of the eye. The lenslet array is conjugate to the pupil plane of the eye in these systems; each lenslet defines the same area in the pupil that it does in the array. A lenslet array with a resolution of 720 μm per lenslet would gather information about the wavefront exiting the eye every 720 μm across the pupil in the vertical and horizontal directions. The system is calibrated with a plano perfect wavefront, providing the focus positions on a CCD camera placed in the focal plane of the lenslet array. When a real eye is measured, any deviation in the horizontal or vertical meridians of each lenslet's measured focal point from the theoretical position can be attributed to the tilt of the light entering the system relative to the optical axis of that lens. By measuring this displacement of the focal point, the tilt of the wavefront for that bundle of rays can be calculated. Combining the information from all of the single lenslets (whose typical dimension is less than 1 mm) of the entire array allows the full wavefront to be reconstructed across the pupil of the optical system.

Fig. 2 shows the Hartmann-Shack image from a human eye. The grid represents the optimal positions of the focal points of the laser beam, whereas the white points are the actual images from each of the lenslets. Based on the deviation of the actual images from their optimal positions, the software algorithm calculates the shape of the wavefront. The Zywave aberrometer in the Zyoptix system uses the Hartman-Shack method of analysis of the outgoing wavefront that measures up to fifth-order Zernike aberrations, including coma, trefoil, and spherical aberrations.

Fig. 2. The Hartmann-Shack image and its derivative wavefront forms.

Use of the Zyoptix system involves multiple steps and calculations. The new Zywave software (version 4.4 of the Zyoptix Evolution Package) for Zyoptix produces a composite aberration map from the best three of five aberration maps. The two outlier maps are discarded. To obtain the wavefront map with the Zywave system, the pupil must first be dilated to at least 6.0 mm. Studies performed at Bausch & Lomb Surgical (Rochester, New York) have found that using a small amount of cycloplegic agent does not adversely affect the wavefront map.

The Orbscan presents an ideal method of obtaining corneal thickness in this setting, because the measurements are obtained optically without any corneal touch, which could otherwise affect the wavefront analysis. The wavefront map is produced and presented in a manner similar to topography maps. This presentation includes a wavefront map on the top left side of the screen, which demonstrates all of the wavefront errors including the second-level Zernike errors of sphere and cylinder. The red color indicates higher-level aberrations, whereas the blue color indicates lower-level aberrations.

The wavefront map on the right side of the screen shows the higher-order root mean square (HORMS) aberrations, which include third-, fourth-, and fifth-order aberrations. This map does not include the spherical and astigmatic components of the wavefront error. The bottom right portion of the screen demonstrates the point-spread function (PSF), or the effect of the aberrations of the visual system on a point source of light. The PSF is helpful as a demonstration tool for patients to illustrate the effect of aberrations on their vision and the potential benefits of custom LASIK surgery. On the Zywave system, the PSF has a standardized scale so it can be used by experienced operators as a qualitative method of evaluating the wavefront error. The PSF at a pupil size of 3 mm is optimized, because the effect of diffraction is balanced by the effect of the wavefront aberration, emphasizing the importance of pupil size in the final analysis. The Optical Society of America has proposed that all wavefront aberrations be reported over 6-mm pupil sizes.

The bottom left side of the screen contains some of the key data points of the wavefront analysis. The total aberrations are recorded as root mean square (RMS). The RMS unit is used to get a sum of the HORMS by adding all of the positive and negative Zernike aberrations to avoid the canceling effect of the negative HORMS on the positive ones. The HORMS are recorded below the RMS value. The RMS is the most important value on the aberration screen, because it represents the sum of the HORMS that the Zyoptix LASIK aims to correct. A preoperative HORMS value of 0.25 to 0.4 μm has been suggested as an indication for Zyoptix LASIK surgery.

The Zywave aberrometer also calculates the PPR, which allows a comparison of the manifest refraction with the refraction generated by the Zywave aberrometer. The PPR is the mean of the sphere and cylinder measurements over a specific pupil size. The PPR values vary according to the pupil size, with the degree of myopia increasing as the pupil dilates. The PPR values are presented with the pupil sizes in one of the Zywave presentation screens. Once the objective Zywave data have been collected, they must be compared with the subjective manifest refraction to ensure consistency. Large differences between the Zywave PPR and the manifest refraction will give suboptimal results for Zyoptix LASIK. The following criteria have been suggested by Bausch & Lomb:

If the PPR differs significantly from the manifest refraction, the test can be repeated while encouraging the patient to relax his or her eyes and look in the direction of the distance target during the testing. If repeated tests yield different results, Zyoptix treatment should not be performed.

 

The Zywave is first measured on the undilated pupil. If the patient accepts the Zywave refraction in the Phoropter, dilated Zywave examinations are performed, and the acceptance of the measurement is checked. If the patient does not accept the Zywave refraction, Zyoptix LASIK is not performed.

Orbscan

The Orbscan is a multidirectional slit-scanning system that collects 9000 data points in 1.5 seconds. The New Orbscan IIz has an upgraded microprocessor that speeds up the calculation time. The Zyoptix mode of the Orbscan IIz must be selected for a minimum level of repeatability and irregularity to be met for the acquisition. A single examination result or an average of multiple examinations can be used. The Orbscan must be performed with an undilated pupil; therefore, it is usually performed before the dilated Zywave map.

The Orbscan Quad map of the cornea provides three-dimensional information about the cornea. The anterior float map demonstrates the anterior elevation of the cornea. The posterior float map demonstrates the posterior elevation of the cornea and becomes important in diagnosing patients with preoperative posterior keratoconus or post-LASIK corneal ectasia, who are not qualified as candidates for LASIK. Generally, greater than 50 μm of deviation from the normal posterior elevation of the cornea is indicated by a red color on the map and should be considered a contraindication for surgery. The refractive map is similar to the keratometric map created with most topography units. The pachymetry map demonstrates the corneal thickness through the entire cornea as well as the thinnest point of the cornea. This data map offers advantages over central corneal ultrasound, because the central portion of the cornea may not be the thinnest point. By reviewing the four color maps together, the clinician can create a mental three-dimensional reconstruction of the cornea.

Zylink software

The Zylink software combines the data of the Zywave and the Orbscan, completing the Zyoptix platform for laser treatment (Fig. 3.). The Zylink software allows the operator to adjust the amount of the sphere and cylinder of the refraction and to set the optical zones of the ablation program. The ablation zone is usually at least the size of the pupil diameter in dim light as measured by Colvard pupillometry. The most commonly desired optical zone is 6.5 mm. Bausch & Lomb suggests an optical zone of at least 0.2 to 0.5 mm more than the scotopic pupil size. The principles of lamellar refractive surgery apply wherein the residual stromal bed should maintain a thickness of at least 250 μm.

Fig. 3. The tetrad of the Orbscan, Zywave, Zylink, and T217Z forming the interactive Zyoptix platform. (Courtesy of Baush & Lomb.)

In standard LASIK surgery, ablation times range from 30 to 40 seconds for a −6.0 D correction with a 6.0-mm optical zone. With the Zyoptix system, which performs four steps at a 2-mm spot size and then goes into a 1-mm spot size for another four to eight steps, this prescription and zone would take about a minute and a half. If an even larger optical zone is used, such as 7.0 mm, the ablation times increases substantially. The Zylink software produces a custom ablation profile window that includes the pachymetry map, the anterior keratometric map, and the customized ablation map, along with the treatment data.

Technolas 217Z laser

The custom treatment file is saved onto a 3.5-in floppy disk, which is fed into the drive of the Technolas 217Z laser. The Zyoptix option is selected from the treatment menu and the special card inserted (Fig. 4). A centration check is performed to ensure that the 1-mm and the 2-mm spots are centered. If the spots are not aligned, the device indicates incorrect card alignment in the laser, and a new card should be used. A fluence test is then performed. Because the laser beam is now passed through the card, the energy level of the laser will need to be increased from the level used for conventional LASIK surgery. Once the 65-pulse breakthrough is obtained, the procedure can be performed.

Fig. 4. The special Zyoptix treatment card. (Courtesy of Bausch & Lomb.)

The Zyoptix card allows the beam profile of the laser to be modified. Most excimer lasers use a Gaussian beam profile, which provides a smooth ablation surface; however, the effective beam diameter for each pulse is dependant on the hydration of the cornea. The flat top beam has a consistent diameter with different degrees of hydration; however, it does not produce a smooth surface. The Zyoptix system uses a combination of the two beam shapes, called a “truncated Gaussian beam,” to achieve the benefits of both beam types (Fig. 5). The 2-mm beam allows most of the refractive error to be corrected in an efficient manner. The 1-mm beam is then used for the more specific ablation patterns and transition zones.

Fig. 5. The truncated Gaussian beam combines the advantages of the flat top and Gaussian beams.

The Technolas 217Z laser has a video-based infrared eyetracker with a frequency of tracking from 60 to 120 Hz. This eyetracker is advantageous, because it does not require pupillary dilation for tracking, which helps pupillary centration and offers greater patient convenience. The eyetracker marks the tracked center of the pupil, which allows adjustment during the procedure. Similarly, the z-axis (height) is also marked on the cornea and can be monitored during the procedure. Instruments can be used during the tracking mode without damaging or disengaging the tracker, which can occur with other systems.

The dynamic eye-tracking module has been added to the 217Z laser. This module recognizes whether the eye movements are too fast for the tracker, and interrupts the laser pulse sequence. The frequency of the tracker is 120 Hz, resulting in an 8.3-ms interval between each capture. The dynamic eye-tracking software will allow 200 μm of movement over the 8.3 ms, which equals 24 μm/ms. Because an ocular saccade occurs at about 25 μm/ms, all fast eye movements are detected. If more than 200 μm of movement occurs, the next laser pulse is interrupted. By applying this technology, the degree of error in the eye tracker is reduced. Using a scanning and flying spot technology, the Technolas 217Z (Table 1) allows surgeons to treat up to −12.00 D of nearsightedness, up to +6.00 D of farsightedness, and up to 5.00 D of astigmatism.

Having been fortunate to use most of the excimer laser systems, the author (ACG) agrees with Slade [1] that the T 217Z platform is ergonomically friendly for the LASIK surgeon. The system has a perfect working distance and offers superior magnification along with an excellent depth of field. The bed is also stable and well designed. There is no distortion of the z-axis, and, above all, the Zyoptix system is a truly excellent workstation.